1. Field of the Invention
The present invention relates to a process of cladding by welding which is advantageous in suppressing defects of cladding.
2. Description of the Related Art
Japanese unexamined patent publication 6 (1994)-257,505discloses a conventional process. Using a cylinder head formed of aluminum alloy having a cladding-scheduled portion for constituting a valve seat, this conventional process irradiates the cladding-scheduled portion with a laser beam to remelt and resolidify all of the surface of the cladding-scheduled portion. After that, this conventional process covers the resolidified portion with a clad layer. Even when the cladding-scheduled portion involves gas therein, this conventional process can release and eliminate gas so as to leave less gas in the cladding-scheduled portion before cladding. Accordingly, this conventional process can suppress expansion of gas in cladding the cladding-scheduled portion by welding.
Japanese unexamined patent publication 64 (1989)-62,282 discloses another conventional process. This another conventional process forms a bead layer by high-frequency welding to produce a seam-welded ferrous pipe. This process irradiates the bead layer with a laser beam after immediately welding, so as to remelt and eliminate weld-defects remaining in the bead layer. In this case, the bead layer is irradiated with the laser beam in such a manner that the bead layer causes little or no molten metal. This process also discloses a technique which detects plasma light generated in eliminating the weld-defects so as to examine the presence of weld-defects in the bead layer.
Japanese unexamined patent publication 6 (1994)-88,120 discloses still another conventional process concerning apparatuses which is to be employed in high temperatures, such as boilers or turbines. This process forms a bead layer by welding, and then irradiates a heat-affected zone in the vicinity of the welded bead layer with a laser beam or an electron beam so as to perform an eliminating step for eliminating the creep-voids remaining in the heat-affected zone.
According to the abovesaid publication 6 (1994)-257,505, almost all of the cladding-scheduled portion constitutes an exposed normal region having a normal surface: the cladding-scheduled portion has very few portions containing gas. In spite of this, this publication technique remelts all of the surface of the cladding-scheduled portion to generate molten metal at the all surface. Accordingly, due to such molten metal and the resultant solidification, this publication technique causes a problem that roughness is generated in all of the surface of the cladding-scheduled portion, thereby causing a problem that the clad layer is unevenness in thickness. Further, remelting all of the surface of the cladding-scheduled portion, this publication technique largely heat-affects the base material constituting the workpiece to rise a problem that balance collapses in strength of the workpiece.
Also, some workpieces have a condition that the cladding-scheduled portion is restricted to be small in thickness. In such workpieces, this publication technique, remelting all of the surface of the cladding-scheduled portion, might cause a problem that some holes unexpectedly are formed in the cladding-scheduled portion. This reason is that control is not necessarily easy in the remelting as production technology.
Since the aforesaid publication technique 64 (1989)-62,282 irradiates all of the surface of the welded bead layer with the laser beam after producing the welded bead layer constituting a clad layer, this publication technique sometimes causes a problem that metallic structure is transformed or deteriorated in all of the surface of the welded bead layer.
Also, since the aforesaid publication technique 6 (1994)-88,120performs the eliminating step after forming the welded bead layer, this publication technique sometimes causes a problem that metallic structure is transformed or deteriorated in all of the welded bead layer.